Copper pyrophosphate electroplating baths



United States Patent Ofiice 3,1575% Patented Nov. 17, 1964 3,157,586 1391*?53 FY13 ll-lGSiHATE ELEiITRfi- PLATEIG Wells, llrdington, and Jerelr Martin England, assignors to Alhright @ldhury, Birmingham, Warh compwy This invention relates to improvements in or relating to copper pyrophosphate electroplating baths and is a modification of the invention of our copending US. application No. 125,363.

According to conventional methods, copper is electrodeposited from an acid bath such as a copper sulpha'te/ sulphuric acid bath or from a cyanide/sodium or potassium cyanide bath. In the latter bath the sodium or potassium cyanide is present in excess so that all the copper is complexed to form the double cyanide. This excess is known as free cyanide. The cyanide type of bath gives rise to health hazards and there is also an effluent disposal problem. Both types of bath have the further disadvantage that smooth thick deposits are dificult to obtain. As electrolysis proceeds and the copper deposit thickens, the surface of the deposit becomes dull and noduled. Indeed with such baths the deposit may be dull and noduled at a thickness of only (1001-0002 inch.

It is also known that copper may be electrodeposited from a pyrophosphate bath. In this instance much thicker deposits, usually live to ten times as thick as deposits obtained'from the acid or cyanide baths, can be obtained before dulling and noduling of the surface starts. For example, when using the following electrolyte, noduling does not occur until a deposit, about 0.01 inch thick, is obtained:

Copper pyrophosphate (hydrated) 85-105 Potassium citrate 25 In use the preferred pH for such a bath is 8.0-9.5, though electrodeposition can occur from such a bath at pH values from 3 to ll, the current density is up to 80 amps/sq. ft. the temperature is 55-60 C. and the bath is vigorously air-a itated. However, such a bath produces only semi-bright deposits.

In our copending application No. 125,363 We describe how bright, smooth, thick deposits'can be produced from copper pyrophosphate electroplating baths it there is present in the bath a smml amount of a compound containing a S-mercapto-l :3 :4-thiadiaz-2-yl radical as brightness additive.

We have now found that Z-mercaptothiazole and hydrocarbon derivat ves thereof may replace the thiadiazyl compounds used in our earlier invention as brightness additives in copper pyrophosphate electroplating baths for the production of bright, smooth, thick deposits.

The present invention therefore provides an aqueous solution for electroplating metal substrates with copper which contains dissolved therein a copper salt, an alkali metal pyrophosphate in an amount at least sufiicient to form the complex X Cu(P O where X represents an alkali metal, characterised in that the solution also contains from 0.5 part per million to saturation of a brightness additive whichis Z-mercaptothiazole in the tree or combined state or a hydrocarbon derivative .thereof; preferably the molecular ratio of pyrophosphate to cop er in the solution is more than 2.0:1 but not more than 2.5 :1.

The invention also comprises solid compositions for solution in water to provide a solution according to the invention. The invention further comprises a method of electroplating metal substrate with copper which comprises immersing the article in a solution of the invention and passing an electric current therethrough with the article as cathode; and also comprises the copper-plated substrates so produced.

In general, the electrolyte solution is prepared by dissolving anhydrous tetra sodium or tetra potassium pyrophosphate in water and then adding the calculated amount of copper pyrophosphate so that the concentration of alkali metal pyrophosphatc is from 250 to 300 g./l. of tetra sodium pyrophosphate or from 300 to 400 g./l. of tetra potassium pyrophosphate and the concentration of the copper pyrophosphate is from to g./l.

Copper may be electroplated from such a bath onto all the usual basis metals and furthermore, because of its low alkalinity, the electrolyte does not readily attack light metals such as aluminium and zinc. It is possible, therefore, to obtain smooth bright deposits of copper on these light metals. In addition dense adherent deposits of copper may be produced on heavier metals, for example iron, steel, copper and copper alloys, nickel and lead.

- This type of bath also has the advantage that fume extraction is not necessary as with the cyanide bath.

The mercaptothiazole brightness additive'may be incorporated in the present solutions in the free or combined state, that is as such or as a salt thereof, particularly in the form of a copper or alkali metal salt, or in the form or" a precursor, such as a bis-thiazyl sulphide 0r disulphide, which can break down to form the mercaptothiazole or a salt thereof under the alkaline conditions prevailing in the solution. Particularly suitable hydrocarbon derivatives of Z-mcrcaptothiazole are 2-mercaptobenzthiazole and alkyl-substituted derivatives thereof, especially 6-alkyl-Z-mercaptobenzthiazoles in which the alkyl group contains from 1 to 4 carbon atoms.

The most effective concentration of the brightness additive in the solution for use in electroplating is from 5 to 10 parts per million, though improved copper deposits are obtained at concentrations as low as 0.5 part per million and the additives may also be used in excess up to the saturation level of concentration.

The electrolyte solution of the invention desirably also contains small quantities of other additives which provide an improved electrolyte process. Thus ammonia may be added to provide a concentration of from 1 to 3 g./l. in order to promote anode dissolution; likewise aliphatic carboxylic acids such as citric acid, tartaric acid or oxalic acid. Also, the addition of alpha-amino acids may be made. These carboxylic acids have the effect of increasing the current density range from which electro deposits of good appearance and mechanical properties may be obtained. The useful current density range may be further extended by additions of nitric acid, nitrate or nitrite ions. The nitrite or nitrate ions are usually added in the form of potassium or sodium salts. Such additives are normally added to the bath after dissolution of the copper salt and pyrophosphate. The thiazole additive is often added as a solution in an alkali metal hydroxide.

In use the electrolyte may be suitably contained in a rubber-lined mild steel tank fitted with a stainless steel steam-heating coil and a plastic ainagitation coil. The concentrations of the bath constituents may vary over a wide range: the higher the metal content of the bath, the

bath desirably between 7 and 11, normally between 7.5 and 9.5 and preferably from 8.6 to 8.9. The pH may be additive is in an amountbetween 5 and parts per 2 million.

adjusted by adding pyrophosphoric acid, nitric acid, sodium hydroxide, potassium hydroxide or ammonia solution. In .use the temperature range of the bath is 35-60 C. and a suitable current density is up to 8G amps/sq. ft.,

though with irregularly shaped articles as cathode it is.

Example 1 In this example copper was plated on to a stainless steel cylinder and the bath composition was as follows:

Copper pyro hosphate (hydrated) g./l. e 100 V Tetra potassium pyrophosphate (anhydrous) g./l. 3.89 Ammonia g./l 2.0

Z-mercaptobenzthiazole "ppm." 5 j The pH of the bath was 8.7, the current density em ployed was 35 amps/sq. ft., the temperature was 60 C. and the bath was agitated by means of air and a rotating cathode.

. Under the above conditions a deposit approximately. inch thick was obtained after 100 hours of plating.

This deposit was bright, level and smooth and was an excellent example of electroforming.

Example 2,

The electroplating process of Example 1 was repeated, but with the plating time reduced to mmutes. Ari electrodeposit of copper having a thickness shghtly greater than 0.0005 inch was formed. This electrodeposit was an excellent tuidercoat for subsequent electrodeposition of nickel or chromium in the formation of decorative articles.

Example 3 In this example a small brass cylinder was made cathodic in a bath containing copper at aconcentration or" g./l., pyrophosphate at a concentration of 200 g./l., ammonia at a concentration of 1 g./l. and 5 ppm. of

- 6-methyl-2-rnercaptobenzthiazole. The pH of the bath was 8.8 and it was operated at 5 5 C.

The brass cylinder was then electroplated at a, current I density of 30 amps per square foot for 2 hours. There was obtained a bright smooth copper electrodeposit on thecylinder ofthickness 0.003 inch which was an entirely satisfactory example of electroforming.

What we claim is:

1. An aqueous solution for electroplating metal substrates with copper which contains dissolved therein: a copper saltj an alkali metal pyrophosphate in an amount at least sufllcient to form the complex X Cu(l 0 wherein X represents an alkali metal; and from 0.5 part per million to saturation of a brightnessadditive which is selected from the group consisting' of 2-mercaptothiazole and 'Z-mercetptobenzthiazole in which the heniz'ene moiety may be substituted with at least one alkyl group having '1 to l carbon'atoms.

2. An aqueous solution'as claimed in claim 1, which i also contains at least one further additiveselected from the group consisting of ammonia in a concentration of analpha-amino acid, nitriteions and nitrateions;

3. Theisolution' of claim" 'wher'ein'said brightness,

With the latter little or no mechaniention may be used 7 4. The solution of claim 1 wherein said brightness additive is a 6-alkyl-2mercaptobenzthiazole in which the alkyl group containsfrom 1 to 4.carbon atoms.

5. The solution of claim 1 wherein said brightness dditive is Z-mercaptobenZthiazole.

6. The solution of claim 1 wherein said brightness";

additive is -methyl-Z-mercaptobenzthiazole.

.7. An aqueous solution for electroplating metal sub- 7 strates with copper which has a pH of from 7 to 13 and contains dissolved thcreinf a copper salt; an alkali metal pyrophosphate selected from the group consisting of sodium and potassium pyrophosphates in an amount sufficient to provide a molar ratio of pyrophosphate to a copper of more than 2;O:1 but not more than 2.5 :1 and a concentration of copper pyrophosphate of from 85 to 100 grams per litre; and from 0.5 part per million to saturation of a brightness additive which is selected from the group consisting'of Z-mercaptothiazole and Z- nercaptobenzthiazole in which the benzene moiety may be substitute d withat' least one alkyl group having 1 to 4 carbon atoms. 2

8. An aqueous solution as claimed in claim 7, which has a pH of from 7.5 to 9.5.

9. An aqueous solution as claimed in claim 7, which also contains at least one further additive selected from the group consisting of ammonia in a concentration of from 1 to 3 grams per litre, an aliphatic carboxylic acid, an alpha-amino acid, nitrite ions and nitrate ions.

H). An aqueous solution for el ctroplatingimetal substrates with copper which contains dissolved therein: a;

copper salt; an alkali metal pyrophosphate in an amount at least suilicient toform the complex X Cu(l" 0' wherein X represents. an alkali metal; and frorn' tlj part per million to saturation of a'brightness additive WhlCl1 is selected from the group consisting of Z-mercaptobenzthiazole' and Z-mercaptobenzthiazole in which the benzene moiety may be substituted with at least one alkyl group-having l to 4 carbon atoms.

11. An aqueous solution for electroplating metal sub- 'strates with copper which has apH or" from 7 to 13 and contains dissolved therein: a copper salt; an alkali metal pyrophosphate selected from the group consisting of sodium and potassium pyrophosphates in an amount sufficient to provide a molar ratio of pyrophosphate to copper of more than 2.021 but not more than 2.5 :1' and a concentration of from to grams per litre; and from 8.5 part per million to saturation of a brightness additive which is selected from the group consisting of 2-mercaptobenzthiazole and Z-mercaptohenzthiazole in which the benzene moiety maybe substituted with at least one alkyl group having 1 to 4carbon atoms.

12. An aqueous solution as claimed in claim has a pH of from 7.5 to 9.5. V I

13. An aqueous solution as claimed in claim 11, which also contains at least one further additive selected from the group consisting of ammonia ina concentration of from 1 to 3 grams per litre, an aliphatic carbox'ylic acid, an alpha-amino acid, nitrite ions and nitrate ions.

14. The solution of claim 13 wherein said brightness 11, which additive is a 6-alkyl-2-mercaptobenzthiazole in which the from 1 to 3 grams per litre, an aliphatic carbo Xylic acid,

alkyl group contains from l'to 4 carbon atoms.

15. The solution of claim 13 wherein said brightness additive is 2-mercaptobenzthiazole.

7 16. The solution of claim 13 wherein said brightness additive is G-methyl-Z-mercaptobenzthiazole.

17. A solid composition consisting essentially of watersoluble copper 'salt;-an alkali metal pyrophosphate in an amount at least sufiicie'nt to form the complex ac trz ni" where X repres nts alkali metal; and a brightness selected from the group consisting ot additive which is zole and Z-mercaptobenzthiazole in which the benzene moiety may be substituted with at least one alkyl group having 1 to 4 carbon atoms, the said composition being water soluble to provide an aqueous solution suitable for use in electroplating metal substrates with copper, and containing from 0.5 part per million to saturation of said brightness additive.

18. A solid composition as claimed in claim 17, wherein the said alkali metal pyrophosphate is selected from the group consisting of sodium and potassium pyrophos- 1O phates and is present in an amount sufiicient to provide a molar ratio of pyrophosphate to copper of more than 2.021 but not more than 2.5 :1.

5 ness additive is Z-mercaptobenzthiazole.

21. The composition of claim 17 wherein said brightness additive is 6-methy1-2-mercaptobenzthiazole.

References Cited in the file of this patent UNITED STATES PATENTS 2,493,692 Stareck- Ian. 3, 1950 2,609,339 Passal Sept. 2, 1952 2,663,684 Pierce Dec. 22, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noe 3,157,586 November 1'1, 1964 Frederick Herbert Wells et ale It is hereby certified that error appears in the above numbered patent reqiiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 48, for "85 part per million" read 0.5 part per million column 4, line 75 and column 5., .line 1, strike out "2-mercaptothiazole in the free state, 2- mercaptothiazole and 2-mercaptobenzthiazole in which the benzene" and insert instead 2-mercaptothiazo1e and 2- mercaptobenzthiazole in which the benzene Signed and sealed this 13th day of April 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Altesting Officer Commissioner of Patents 

1. AN AQUEOUS SOLUTION FOR ELECTROPLATING METAL SUBSTRATES WITH COPPER WHICH CONTAINS DISSOLVED THEREIN; A COPPER SALT; AN ALKALI METAL PYROPHOSPHATE IN AN AMOUNT AT LEAST SUFFICIENT TO FORM THE COMPLEX X6CU(P2O7)2, WHEREIN X REPRESENTS AND ALKALI METAL; AND FROM 0.5 PART PER MILLLION TO SATURATION OF A BRIGHTNESS ADDITIVE WHICH IS SELECTED FROM THE GROUP CONSISTING OF 2-MERCAPTOTHIAZOLE AND 2-MERCAPTOBENZITHIAZOLE IN WHICH THE BENZENE MOIETY MAY BE SUBSTITUTED WITH AT LEAST ONE ALKYL GROUP HAVING 1 TO 4 CARBON ATOMS. 